Sensors which respond to changes in inductive reactance are well-known. Such sensors may include one or more relatively stationary elements such as a coil of wire that constitutes an inductor characterized by inductive impedance or reactance, and a movable member which moves in the field or interacts with the stationary part in accordance with the physical position of the moving part to change the inductive reactance of the electrical connections to the sensor elements. Among these are eddy current type inductive sensors where the variation in inductive reactance and the variation in the effective series resistance of the sensor are related to the position of a moving conductive cylinder or spoiler.
Such sensors are especially useful because they are not typically subject to wear as are those sensors in which the moving and stationary parts are in contact. For example, in a simple potentiometer having a wiper blade which moves along and contacts a resistance winding, the constant moving, frictional contact between the wiper blade and the resistance winding will cause wear on the parts and so limit the useful lifetime and long term accuracy of the apparatus. Such sensors are also useful for measurements made in very high temperature environments. A coil of wire can easily be fashioned in a way so as to withstand extremely high temperatures. Moreover, such sensors are inherently very reliable on account of their simple construction involving only a single length of wire and the requirement of only two electrical connections to the sensor element.
The utility of such inductive sensors can be furthered in connection with electronic circuits that connect such sensors to a fixed capacitor to form a resonant circuit. Benefits of such a resonant circuit arrangement follow from the ability of such resonant circuit elements to store and accumulate energy in alternating electric and magnetic fields in the two reactive circuit elements of the circuit. Because of this ability to accumulate the excitation necessary to develop a useful signal level a sensor indication can be developed with lower power consumption.
Moreover, there are also advantages in designing inductive sensors that work with higher frequencies of electrical excitation. This is so because as the reactive impedance of the inductor increases in direct proportion to the frequency of operation. The advantages of operating inductive sensors at higher frequencies include lower operation power requirements, lower cost of manufacture due to fewer turns of wire required to develop the necessary inductance, and faster and more accurate position measurement response because of the higher frequency of the information signal.
Operating such sensors at higher frequencies is not without compromise however. Implementing inductive sensor circuits that operate at higher frequencies and in conjunction with a capacitor often result in sensitivity of the position function to various extraneous capacitances. These extraneous capacitances can include the capacitance in lead wires to the sensor, the equivalent capacitive couplings to the sensor element between the sensor and its surroundings, and even the reflection of the oscillator circuit gain's lag time in the input of the oscillator's gain stage.